In the tobacco industry, various types of filter devices have been developed which permit the blending of air and cigarette smoke within the filter itself. A common type of filter device consists of two plugs of fibrous material surrounding a cavity in which charcoal is situated. The filter plug-charcoal cavity-filter plug assembly (the "plug assembly") is assembled as follows. A sheet of machine-perforated paper, known as a combiner wrap, is machine-rolled with a quantity of heated hot melt adhesive. While the adhesive is still hot (and therefore fluid), the filter plugs are anchored to the combiner wrap and a quantity of charcoal is placed in the cavity between the two filter plugs. The combiner wrap is of a sufficient stiffness to prevent collapse of the charcoal cavity.
The perforations of the combiner wrap are holes which are basically straight-line cavities extending the entire thickness of the paper. When the hot melt adhesive is rolled onto the inner surface of the perforated combiner wrap, the adhesive tends to penetrate through the perforations and onto the outer surface of the combiner wrap. This limits ventilation level and causes a problem when the tipping process is initiated. The plug assembly is attached to the cigarette rod (a tobacco column wrapped with paper) by a tipping paper. The tipping paper is adhered to both the cigarette paper and the plug assembly by an aqueous adhesive. If the hot melt adhesive has soaked through the combiner wrap to the outer surface, which is adjacent to the tipping paper, the tipping paper's adhesive will not bond as effectively to the combiner wrap. This interference creates a ventilation problem within the cigarette. The tipping process is well known in the art, as illustrated in U.S. Pat. Nos. 4,295,478; 4,411,279; and 4,480,644, all of which are incorporated herein by reference.
In addition, the rolling of hot melt adhesive onto perforated paper results in the use of more adhesive than is actually necessary to properly seal the plug assembly, as adhesive not at the paper's surface is unavailable for sealing. In other industries, such as the disposable diaper industry, adhesive is sprayed onto surfaces. However, unlike the tobacco industry, the disposable diaper industry is not concerned with air ventilation.
It is well known in the industry to use naturally porous paper for the combiner wrap instead of artificially perforated paper, as illustrated in United Kingdom Patent No. 2099678B, also incorporated herein by reference. An advantage of naturally porous paper is that the pores are not straight-line holes extending the entire thickness of the paper. Instead, the pore form randomly shaped, circuitous paths stretching from one surface of the paper to the other. However, the method of rolling hot melt adhesive onto porous, as opposed to perforated, combiner wrap paper, still results in the penetration of adhesive into the combiner wrap paper, leaving the above-mentioned problems of ventilation and wasted adhesive unsolved.
Accordingly, there is a need in the art for an improved method of applying adhesive to combiner wrap paper such that the adhesive does not penetrate into the paper and interfere with the tipping process.
There is also a need in the art to provide an improved cigarette filter which uses only the requisite quantity of hot melt adhesive.